Add a footer line to the response. The strings referenced by
footer and content must be zero-terminated and they are
duplicated into memory blocks embedded in response.

Notice that the strings must not hold newlines, carriage returns or tab
chars. You can add response footers at any time before signalling the
end of the response to MHD (not just before calling 'MHD_queue_response').
Footers are useful for adding cryptographic checksums to the reply or to
signal errors encountered during data generation. This call was introduced
in MHD 0.9.3.